The present invention relates to a new and improved pneumatic vehicle tire having a strand-like tread member or tread surface comprising a rubber mixture which contains a predetermined proportion of plasticizer.
The present invention also relates to a new and improved plasticizer mixture for use in a tread member rubber mixture of a pneumatic vehicle tire as well as to a novel method of preparing such plasticizer mixture.
In the construction of tires, a predominant effort is expended in improving to the greatest extent possible the tire travelling characteristics or properties independent of the effects of the environment and climatic conditions. In particular, it is intended to positively affect criteria tire characteristics, such as gripping force, skid resistance, rolling resistance and the like, which are essential in this connection, by correspondingly further developing the tire profile and, above all, also the tread member rubber mixture or compound.
Polymers having low glass transition temperature like natural rubber (NR) and cis-polybutadiene rubber (BR) are known to provide good traction on snow and ice; see the article entitled "The Friction of Polymers on Ice", authored by W. Gnorich and K. A. Grosch, appearing in the Journal of The Institution of the Rubber Industry 6, 192-196 (1972). However, in such blends or formulations the wet tire adherence or traction is unsatisfactory.
Starting from these conventional tread member rubber mixtures or compounds and in order to improve upon their property balance under ice, snow, dry and wet conditions, the materials of the tread surface have been further developed by selecting polymers and polymer blends having higher damping and, therefore, positive effects on wet traction or adherence, on the one hand, and by using plasticizers which reduce hardening of the rubber at low temperatures, on the other hand. Since usually the winter and wet travelling properties work in opposition to each other, polymers which improve the wet traction, however, simultaneously will result in a deterioration in snow and ice traction in most cases. The addition of cold condition plasticizers (see the paper of Asahiro Ahagon et al., entitled "Friction on Ice", presented at the meeting of the Rubber Division, American Chemical Society, 131st Meeting, Montreal, Quebec, Canada, May 26-29, 1987), such as naphthenic or paraffinic mineral oil plasticizers or ester plasticizing agents, while improving upon the winter properties, simultaneously result in deteriorated wet traction.
According to another prior art proposal, it is contended that, by conjointly adding butyl rubber and ester plasticizing agents, the gripping force, which is effective on snow and ice, is improved without reducing the gripping force on wet roads, see, for example, German Patent No. 3,417,166, granted Feb. 28, 1985. However, disadvantages thereof reside in a deterioration of the properties on dry roads as well as an increased crack sensitivity, which is caused by the incompatibility of the aforementioned rubbers and butyl rubber, and the high price of the mentioned materials, namely the butyl rubber and the ester compounds. Also, the mixing process tends to be difficult due to the aforementioned incompatibility because inhomogeneities become unavoidable.